Data are analyzed using the well-known Clausius Clapeyron equation, under the assumption of constant enthalpy of sublimation over the temperature ranges measured. The vapor pressure of the pure compound, P°, is related to the enthalpy, Δsub
H, and entropy, Δsub
S, of sublimation via:
summarizes the compounds used in this investigation and the sublimation enthalpies and entropies obtained for each compound for their respective measured temperature ranges. A 95% confidence interval was calculated for each set of reported values via linear regression. The last column in is the vapor pressure, extrapolated using the Clausius-Clapeyron equation (equation 2
, above) to ambient temperature, 298K to provide a rough sense of the relative volatilities of the compounds studied. presents the raw vapor pressure data obtained using the Knudsen effusion technique; these data form the basis of the results in .
Compounds investigated and results obtained from vapor pressure measurements on oxygenated polycyclic aromatic hydrocarbons.
Raw vapor pressure data Pvap (in Pa) obtained for pure OPAHs as a function of temperature (T, in Kelvin) using the Knudsen effusion technique
through demonstrate the effect adding an oxygen-containing heteroatom to polycyclic aromatic hydrocarbons; in each case, the vapor pressure of the parent compound decreases and enthalpy of sublimation increases upon addition.
Figure 1 Vapor pressure of 1-bromo-2-naphthoic acid compared to parent and relevant polycyclic aromatic compounds, as measured by the Knudsen effusion technique plotted as the natural log of pressure (ln Pvap versus reciprocal of temperature (T); ν naphthalene (more ...)
Figure 4 Vapor pressures of oxygenated anthracene compared to parent PAH as measured by the Knudsen effusion technique; ν anthracene; λ 9-anthraldehyde; υ 2-anthracenecarboxylic acid; σ 9-anthracenecarboxylic acid
demonstrates the effect of adding both a brominated and oxygenated heteroatom on the thermodynamics of naphthalene. The vapor pressure of 1-bromo-2-naphthoic acid is more than six orders of magnitude less than that of pure naphthalene, whereas the addition of one bromine only decreases the vapor pressure by one order of magnitude [17
]. The enthalpy of sublimation of naphthalene, increased by 7.1kJ/mole with the addition of one bromine, and increases 35.7kJ/mole from naphthalene to 1-bromo-naphthoic acid. also presents the vapor pressure data on 1- and 2-naphthylacetic acid, where the vapor pressure decreases by five and six orders of magnitude respectively, over the parent compound, naphthalene [19
]. The enthalpy of sublimation of 1-naphthylacetic acid is 53% higher than that for pure naphthalene; it is 70% higher for 2- naphthylacetic acid, alluding to the relative importance of the carbon position of the substituted heteroatom.
and demonstrate the significant impact of a nitro group addition to fluorene and pyrene, respectively. The addition of a nitro group at the 2-carbon position of fluorene increases the enthalpy of sublimation from 88.1(±1.9) kJ/mole to 114.2(±3.0) kJ/mole. For pyrene, a nitro group substituted on the 1-carbon increases the enthalpy from 97.8(±3.3) kJ/mole to 125.0(±3.8) kJ/mole, a comparably similar increase. This trend was also noted in data measured by Ribeiro da Silva et al. [20
] on the vapor pressures of 1-nitronaphthalene and 9-nitroanthracene; adding a nitro group to the former resulted in an increase of 21.8kJ/mole over naphthalene, while addition to the latter yielded an increase of only 16.9kJ/mole over anthracene. Hence, their results showed slightly lower enthalpy contributions than did ours. Adding the nitro group on the 9-carbon of anthracene produced the smallest effect on enthalpy of sublimation a–17% increase. However, the other compounds resulted in increases of 30%, 28%, and 30% for 2-nitrofluorene, 1-nitropyrene, and 1-nitronaphthalene, respectively.
Figure 2 Vapor pressures of oxygenated fluorene compared to parent PAH as measured by the Knudsen effusion technique; ν fluorene ; λ 2-fluorenecarboxaldehyde;υ 9-fluorenecarboxylic acid; σ 2-nitrofluorene
Figure 3 Vapor pressures of oxygenated pyrene compared to parent PAH as measured by the Knudsen effusion technique; ν pyrene; λ 1-pyrenecarboxaldehyde; υ 1-nitropyrene
Previous studies from this laboratory indicated that for halogenated heteroatom substitution onto PAHs, the position of the halogen substituted on the parent molecule does not seem to play a large role in the vapor pressure behavior [16
]. However, as we see here through the anthracenecarboxylic acid structural isomers (), the position of the substituted group on the parent PAH is quite significant; the vapor pressure of 2-anthracenecarboyxlic acid is almost a full order of magnitude less than that of 9-anthracenecarboyxlic acid at ambient temperature. The differences in enthalpy are also significant; we report an enthalpy of sublimation for 2-anthracenecarboyxlic acid of 134.8±3.4 kJ/mole, whereas for 9-anthracenecarboyxlic acid the Δsub
H is 120.1±3.8 kJ/mole. Additionally, as seen through the intercept of the Clausius-Clapeyron equation, the entropy of sublimation for 2-anthracenecarboyxlic acid was calculated as 0.301±0.008 kJ/mole-K, for 9-anthracenecarboyxlic acid to be 0.278±0.009 kJ/mol-K. Thus, we see a slightly larger impact on entropy of sublimation for the 2-anthracenecarboyxlic acid. From these data, we note the significance of the substituent position of the carboxyl group on the parent PAH. Further investigations into this trend are warranted in order to establish whether molecular symmetry (i.e., the carboxyl group on 9-anthracenecarboxylic acid sits on a center carbon of the parent, whereas for 2-anthracenecarboyxlic acid the carboxyl group sits on an end carbon) is an important determinant in a compound’s vapor pressure. Many questions remain as to the implications for potential hydrogen bonding and/or induced dipole moments within the PAH molecule and its oxygenated heteroatoms.
Also in , we see that the addition of a carboxyl group to fluorene at the 9-carbon position increases the enthalpy of sublimation by 21.9kJ/mole, an increase of approximately 25%. We also see a vapor pressure depression of over four orders of magnitude, illustrated in . We expect to see this larger increase in enthalpy of sublimation due to heteroatom substitution on a smaller compound, such as fluorene, than on anthracene.
presents the results of vapor pressure measurements on 9-anthraldehyde. The addition of the aldehyde group as seen in , has a considerably larger impact, increasing the enthalpy of sublimation by 11.9 kJ/mole while decreasing the vapor pressure by almost two orders of magnitude at 298K. A similar impact is seen with 1-pyrenecarboxyaldehyde, where an aldehyde at the 1-carbon position increases the enthalpy of sublimation from 97.8±3.3 kJ/mole to 110.4±3.8 kJ/mole, an increase of almost 13%. Likewise, the vapor pressure is decreased by two orders of magnitude at 298K, demonstrated in .